Solar ammonia process may spur fertiliser revolution

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But a new low-energy method could potentially be deployed in off-grid regions

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“STEP ammonia does not need connection to the electrical grid and hence is suitable for poor and rural areas which have less developed infrastructure.”

Stuart Licht, George Washington University

Low-energy ways to make ammonia — a vital constituent of fertiliser — could enable developing countries to manufacture their own fertiliser instead of importing it.

One such technology published last month (8 August) in Science produces the chemical directly from air, steam and sunlight, and could offer an alternative to the conventional Haber-Bosch method. This relies on high temperatures and pressures, and so requires vast amounts of energy: globally it consumes more than 1 per cent of total energy generated each year.

The new method, developed by Stuart Licht of George Washington University in the United States and his colleagues, makes ammonia by bubbling steam into a cheap molten salt mixed with nanoscale iron particles, and passing an electric current through.

Core to the team’s concept is ‘Solar Thermal Electrochemical Production’ (STEP), a process where sunlight is harvested and used to help power chemical reactions, reducing the need for energy input from other sources. Licht’s team has also developed methods for making cement and treating water using STEP.

Although their published ammonia production method uses mains electricity, solar energy can in principle provide both the electrical current and heat required.

“STEP ammonia does not need connection to the electrical grid and hence is suitable for poor and rural areas which have less developed infrastructure,” Licht says. He adds that any STEP process could be used on a small scale.

Paul Makepeace, a fertiliser specialist at the International Fertilizer Development Center in Nairobi, Kenya, says such an approach “is certainly of interest” in Sub-Saharan Africa.

Currently Nigeria is the only country in the region that makes ammonia, and it does this using the Haber-Bosch process.

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“This method’s use and development will depend on its competitive advantage and the ability to deliver manufacturing capacity to African countries where fertiliser is a significant foreign exchange-spend,” says Makepeace.

But he adds that the STEP process “seems like it has a long way to go” to reach commercialisation. Licht concedes that his group is “not geared towards technology spin-out”, and says he hopes industry or government will fill that role.

Indeed, another electrolytic approach to making ammonia from air and water is due to be commercialised this year, says Makepeace. NHThree, a company based in Richland, United States, is building an ammonia synthesis plant for Bolivia’s Royal Silver Company.

NHThree heats steam to form hydrogen ions and applies an electrical current to drive them across a membrane to react with nitrogen from the air to form ammonia. The company’s method does not directly exploit solar energy, but still uses 30 per cent less energy than the Haber-Bosch method, according to its website.

NHThree’s founder Jason Ganley says the method “will be best suited to sites with plentiful cheap, or off-cycle, renewable power”.

This is because to compete with current technologies it will need very low energy prices, but “there may be opportunities present in Africa such as Ethiopia’s Grand Renaissance Dam, that make this technology viable,” says Makepeace.

“The important thing is to keep our eyes open to this technology,” he adds.